I think it depends on what type of hop/trub filter you plan on using, but I would say 1" to 1 1/2" would allow some options if you don't have one in mind right now. My keggle valve was already installed when I got it and is about 4" off the bottom (too high). My SS brew kettle valve is 1" on center off the bottom (much better for me).

__________________On Deck
perhaps a line of single hop IPA's - there's so many new hops out there!!!